1. Field of the Invention
The present invention relates to a method and means for volumetric expansion testing and, more particularly, to a system for automatically displaying and recording the expansion data from the hydrostatic testing of pressure vessels with an accuracy and repeatability meeting or exceeding existing requirements.
2. Description of the Prior Art
High pressure steel cylinders conventionally are used to contain compressed gases for a wide variety of medical and industrial uses. Literally thousands of such compressed gas cylinders are in use daily to contain under pressure oxygen, nitrogen, hydrogen, argon, and other gases. To minimize the possibility of rupture of such cylinders, with the associated dangers of explosion and/or loss of combustible or poisonous gas, periodic expansion testing to determine the plastic and elastic deformation of the cylinders normally is carried out.
In accordance with U.S. Interstate Commerce Commission regulations, compressed gas cylinders used in interstate commerce must be checked once every five years by subjecting such cylinders to an internal pressure one and two-thirds times the working or rated pressure thereof. Conventionally, this testing is carried out hydrostatically in accordance with one of a few methods so as to accurately obtain total expansion, permanent expansion, and thereby percent permanent expansion. According to the most popular method, a cylinder is filled with water and sealed in a water-filled jacket connected to a burette. High pressure water is introduced into the water-filled cylinder, the pressure being raised to the requisite one and two-thirds times the nominal pressure of the cylinder. The amount of water displaced by expansion of the cylinder is then conducted to the burette so that total expansion may be determined by noting on the burette the amount of water displaced by the jacket as a result of the cylinder expansion.
To measure permanent expansion and percentage permanent expansion, the internal cylinder pressure is released. If the cylinder returns to its original shape, the burette will return to its zero position. If the cylinder does not return to within ten percent of its original shape, as indicated by the burette reading, the cylinder is considered defective. Such a cylinder could rupture when subjected subsequently to severe shock or stress while filled with high pressure compressed gas.
The other commonly used method of hydrostatic testing is the direct expansion method. However, since it has practical limitations in its use, it is used substantially less frequently than the water jacket expansion method. In any event, the direct expansion method consists of forcing a measurable volume of water into a cylinder filled with a known weight of water at a known temperature and measuring the volume of water expelled from the cylinder when pressure is released. The permanent volumetric expansion of the cylinder is calculated by subtracting the volume of water expelled from the volume of water forced into the cylinder. The total volumetric expansion of the cylinder is calculated by subtracting the compressibility of the volume of water in the cylinder, when under pressure, from the volume of water forced into the cylinder to raise the pressure to desired test pressure.
Since there are far fewer problems associated with obtaining accurate data with the water jacket volumetric expansion method, it is the predominant method in use. According to such method, the burette is raised or lowered to maintain the water level at a specific zero level in order to negate any errors caused by changes in hydrostatic head. However, movement of the calibrated glass burette and recording of the pressure vessel expansion values has all been done manually, which is slow and prone to error. The use of a human operator to gather the information also precluded the automation of this process to achieve greater productivity and accuracy.
In order to overcome this problem, suggestions have been made to use level detectors and servo mechanisms to raise and lower the burette, thereby interpreting such movement as expansion values. However, this method has proven to be too complex, requiring too many mechanical components with their attendant maintenance problems.
It has also been suggested to measure the weight of the displaced fluid to determine volumetric changes. However, this concept, too, has substantial problems. For example, it is necessary to establish a zero starting point automatically and repetitively in order to provide the degree of speed and accuracy which is necessary. The removal of residual water from a previous test as well as adding surplus water from the purging of the test jacket fluid expansion line in order to achieve the zero starting point repetitively and accurately requires changes in operating concepts far removed from those used with a calibrated burette. Furthermore, if a sensitive weighing element is to be used, it is necessary to isolate it from extraneous vibration to prevent wide fluctuations in the readings that could lead to inconsistent data. It is also necessary to develop a means of keeping the fluid level as close as possible to its original level as water is added or removed from the collection vessel so as not to introduce any errors caused by changes in hydrostatic head forces on the surfaces of the test jacket.
It is also necessary that the accuracy and repeatability of any system meet or equal the requirements of the Hazardous Materials Regulations of the Department of Transportation published by the Bureau of Explosives of the Association of American Railroads. These regulations state that the gauge indicating the total expansion of the cylinder must be such that the total expansion can be read with an accuracy of 1%, except that a reading of 0.1 cubic centimeters shall be acceptable. While the 1% provides a certain amount of leeway when testing large cylinders, the 0.1 cubic centimeter readability, as is required in the case of small cylinders, requires accuracies and repeatabilities that are difficult to achieve. As a result of these and other problems, no practical means has been developed heretofore for automating volumetric expansion testing so as to achieve greater productivity.